Integral membrane proteins play an important role in a variety of biological processes. In particular, they are a crucial component in electron transport and energy conversion in biological membranes. The goal of the proposed research is to understand the structure and function of a model membrane protein, the reaction center from a photosynthetic purple bacterium that converts light into chemical energy. Several of the electron transfer reactions in this system resemble the reactions that occur in systems of higher organisms such as mitochondria. However, the advantages of studying this model system rather than the mitochondria are its simplicity and the ease of starting the electron transfer process by applying light. The approach to study the structure of the reaction center proteins will be to prepare single crystals of the proteins that are suitable for x-ray diffraction. In addition, we plan to use electron paramagnetic resonance and spectrophotometry (linear dichroism) to supplement the data obtained by x-ray diffraction. Also, we plan to systematically investigate the parameters that lead to successful crystallization. This will provide a better understanding of the basic principles underlying the mechanisms of crystallization and should lead to a rational approach to design crystallization conditions for other membrane proteins of importance to biology.